IV-VI colloidal quantum dot photodetectors integrated on silicon waveguide circuits

Research Area: Heterogeneous integration technology for silicon photonics , Silicon photonics for lab-on-chip spectroscopy

Main Researcher: Chen Hu

Many molecules that we want to detect or monitor in our environment have bands of absorption lines in the mid-infrared (MIR). These absorption lines form a ¡°fingerprint¡± for a particular molecule, hence MIR spectroscopic sensing systems allow detecting the presence (and concentration) of specific molecules (Figure 1). Traditional high-sensitivity photodetectors used in these spectroscopic systems are based on epitaxial materials, leading to expensive devices. Low-cost MidIR photodetectors based on colloidal quantum dots (QDs) offer an alternative way to realize this functionality, either as discrete components or integrated on photonic integrated circuits.

Figure 1. Illustration of the characteristic absorption lines of molecules in mid-infrared range.

Using colloidal QDs as new photonic material gets a lot of attention in the photonic community. The simple hot injection chemical synthesis method allows low-cost production. By tuning the size of the QDs, the electrical and optical properties (such as the absorption cut-off wavelength) can be tuned due to the quantum size effect (Figure 2). Furthermore, the fact that these QDs are available in solution makes it easy to do large-area heterogeneous integration on substrates, using dip coating or printing, which can offer a considerable cost reduction as compared to thermal evaporation or epitaxially grown layer stacks.

Figure 2. Typical CdSe size series, as obtained through hot-injection based synthesis.

In this project we propose to use IV-VI or II-VI colloidal quantum dot materials, such as PbS, PbSe and HgTe, for infrared photodetector application. We demonstrate a uniform, ultra-smooth colloidal QD film without any cracks, which is realized by dip coating and subsequent ligand exchange. Metal-free inorganic ligands, such as OH- and S2-, are investigated to facilitate the charge carrier transport. Both PbS and HgTe-based quantum dot photoconductors were fabricated on interdigitated gold electrodes. For PbS-based detectors a responsivity of 200A/W is measured at 1.5¦Ìm, due to the large internal photoconductive gain. A 2.2¦Ìm cut-off wavelength for PbS photodetectors and 2.8¦Ìm for HgTe quantum dot photodetectors are obtained (Figure 3).

Figure 3. Normalized detector responsivity as a function of wavelength for S2- capped and OH- capped PbS QD photodetectors.

Other people involved:

• Chen Hu

PhD thesises